Ligand-free Dual (Bis) Schiff Base Chitosan-drug Conjugates for pH-responsive Anticancer Drug Delivery: Design Principles, Comparative Performance, and Translational Outlook

Abstract:

Chitosan has been investigated for several decades as a carrier for anticancer agents because of its biocompatibility, biodegradability, chemical versatility, and intrinsic pH responsiveness. Most reported platforms fall into two groups: systems in which the drug is covalently linked to chitosan through a single imine (Schiff base) bond, and ligand-functionalized systems in which folic acid, hyaluronic acid, or galactose is introduced onto the backbone to promote receptor-mediated cellular uptake. Both approaches have been discussed extensively elsewhere. However, a comparatively underexplored strategy involves the formation of a bis-imine linkage between a carbonylcontaining anticancer drug and two amino groups of chitosan. In this configuration, the drug serves not only as the therapeutic payload but also as the sole organic linker between two sites on the polymer chain. This ligand-free dual Schiff base architecture is treated here as a distinct class of chitosandrug conjugate. Both of its defining terms have been applied inconsistently in the literature, and “ligand- free” and “dual (bis) Schiff base” are therefore defined operationally, together with representative examples of structures that fall inside and outside each category. Bivalent attachment is then related to physicochemical and release behavior relative to single-imine systems, with attention to the structural consequences of engaging two amines, the acid-catalyzed hydrolysis of the C═N bond, and the electronic character of the carbonyl-bearing drug. Compared with other acid-labile bonds used in polymer–drug conjugates, the imine is less stable and less pH-selective than the acylhydrazone; however, requiring two hydrolysis events rather than one offers a structural route to improved drug retention. Ligand-modified chitosan systems are assessed critically rather than solely in terms of their targeting advantages, with attention to synthetic complexity, effects on drug loading, steric hindrance, and variability in receptor expression. The tumor-associated pH gradient, on which the whole strategy depends, is weighed against the values actually measured in patients rather than assumed to be a sufficient stimulus for imine hydrolysis and controlled release. Particular emphasis is placed on the analytical evidence required to establish the formation of two distinct imine bonds, since confirmation of a bis-imine structure requires more than indirect evidence of drug association with the polymer. Release evaluation is addressed in terms of physiologically relevant pH conditions, appropriate kinetic models, and transparent reporting of release parameters, and the translational outlook is read against the clinical history of polymer-drug conjugates. A three-way comparison is provided between conventional chitosan-based drug carriers, ligand-functionalized chitosan systems, and ligand-free dual Schiff base chitosan-drug conjugates with respect to synthetic complexity, structural stability, drug-loading potential, release behavior, targeting strategy, and translational considerations. Strictly ligand-free bis-imine chitosan-drug conjugates have been reported in only a small number of studies, so parts of the comparison necessarily draw on closely related imine-linked polymer– drug systems. The resulting experimental gaps are therefore identified explicitly, together with the studies needed to establish the reproducibility, structural stability, release characteristics, biological performance, and safety of these conjugates. Addressing these issues will be essential for determining whether ligand-free dual Schiff base chitosan-drug conjugates can progress beyond proof-ofconcept studies toward more comprehensive preclinical and, ultimately, clinical evaluation.